Carbon nanotubes are seamless tubes of graphite sheets with complete fullerene caps and were first discovered as multi-layer concentric tubes or multi-walled carbon nanotubes, and subsequently, as single-wall carbon nanotubes. Nanotubes are typically formed in the presence of transition metal catalysts. Carbon nanotubes have shown promise in applications such as nanoscale electronic devices, high strength materials, thermally and electrically conducting materials, electron field emission devices, tips for scanning probe microscopy, gas filtration, and gas storage.
For a number of applications, single-wall carbon nanotubes (SWCNTs) are preferred over multi-walled carbon nanotubes, because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes (MWCNTs) of similar length. Defects are less likely to occur in SWCNTs. MWCNTs can survive occasional defects by forming bridges between unsaturated carbon valances, while SWCNTs have no neighboring walls to compensate for such defects.
The availability of carbon nanotubes in quantities necessary for practical technology development and application is problematic. The development of efficient processes for producing carbon nanotubes of consistent high quality in quantity is the key to the commercialization of specialty carbon nanomaterials (CNMs).
Conventional carbon fiber materials and fiberglass are used as additives in composite polymeric materials, for structural reinforcement. Conventional carbon fibers and metal fibers are used as additives in polymers to provide electrical conductive properties required to dissipate static electricity, to provide electromagnetic shielding, and to increase thermal conductivity. Graphite carbon nanofibers have been utilized as a replacement additive for conventional carbon fibers, resulting in improvements in the mechanical and electrical properties of numerous polymer blends. Significant reduction in weight and production costs of finished products has been demonstrated. Although several companies in the conductive plastic industry are starting to incorporate carbon nanofibers in their products, they cite price, product consistency, and supply reliability as major issues. It would therefore be desirable to develop a method and apparatus for cost effectively producing commercial quantities of CNMs.
It is recognized that amorphous carbon nanowires have lower mechanical strength and electrical conduction than carbon nanotubes. However, carbon nanowires have large active surface areas that appear to be beneficial for applications such as ultra-filtration and hydrogen storage. The suitability of carbon nanowires for such applications is currently under investigation.
Presently, there are three main approaches for synthesis of carbon nanotubes. These include the laser ablation of carbon (Thess, A. et al., Science 273:483 (1996)), the electric arc discharge of a graphite rod (Journet, C. et al., Nature 388:756 (1997)), and the chemical vapor deposition (CVD) of hydrocarbons (Qin, L. et al., Appl. Phys. Lett. 72:26 (1998)).
SWCNTs are reported to have been produced at a rate of 10 grams per day by CVD in a high-pressure (30 to 50 atm), high-temperature (900° C. to 1,100° C.) process (HiPco Process), using carbon monoxide (CO) as the carbonaceous precursor material and a liquid catalyst in a small continuous-flow reactor (Bronikowski, M. et al., J. Vac. Sci. Technol. A 19(4), (2001)). Such a technique suffers from the disadvantages of requiring high pressure systems (which significantly increases operating costs), having a production rate that is insufficient to meet the anticipated demand for CNMs, and for being able to utilize only a single feedstock (CO). It would therefore be desirable to provide a method and apparatus for producing CNMs that does not require high pressure systems, that can produce larger quantities of CNMs, and which can use various different feed stocks.
The production of MWCNTs by catalytic hydrocarbon cracking is now being achieved on a commercial scale (see U.S. Pat. No. 5,578,543), while the production of SWCNTs is still only achievable in gram scale quantities by the laser ablation technique (Smiljanic, O. et al., INRS Energie et Materiaux, Canada, Sa-PS2-Sy27, Log No. P109, (2002)) and arc discharge technique. Both the laser ablation method and the arc discharge method suffer from being difficult to implement as large quantity production processes (Zheng, B. et al., Appl. Phys. A74:345-348 (2002)). New and refined techniques for SWCNTs production are in the introduction phase (Resasco et al., U.S. Pat. No. 6,333,016).
CVD over transition metal catalysts (on-substrate method) has produced both MWCNTs and SWCNTs. The catalyst selection and surface preparation strongly influence the CNM morphology. Laser ablation, arc techniques, and the catalytic hydrocarbon cracking process can be used for the production of SWCNTs. Dai et al. demonstrated web-like SWCNTs resulting from the disproportionation of carbon monoxide (CO) with a molybdenum (Mo) catalyst supported on alumina, heated to 1200° C. From the reported electron microscope images, the Mo metal apparently attaches to the nanotubes at their tips. The reported diameter of SWCNTs generally varies from 1 nm to 5 nm, and seems to be controlled by the particle size of the Mo catalyst. Catalysts containing iron, cobalt, or nickel have been used at temperatures between 850° C. to 1200° C., to form MWCNTs (U.S. Pat. No. 4,663,230). Rope-like bundles of SWCNTs have been generated during the thermal cracking of benzene with an iron catalyst and sulfur additives, at temperatures between 1100° C.-1200° C. The synthesized SWCNTs are roughly aligned in bundles and woven together like those obtained from the laser ablation and electric arc methods.
Vaporizing targets, including one or more Group VI or Group VIII transition metals, and graphite using lasers to form SWCNTs have been proposed. The use of metal catalysts, including iron and at least one element selected from Groups V (V, Nb, and Ta), VI (Cr, Mo, and W), VII (Mn, Tc, and Re), or the lanthanides, has also been proposed (see U.S. Pat. No. 5,707,916). Recently, new methods have been proposed that use catalysts to produce quantities of nanotubes having a high ratio of SWCNTs to MWCNTs (Resasco et al., U.S. Pat. No. 6,333,016).
As applications for graphite carbon nanotubes, carbon nanofibers, and amorphous carbon nanowires develop, the demand for these products will grow. Market introduction of CNM for producing products and in other applications is highly dependent on the availability of cost effective production methods.
The majority of the processes described above involve growing the CNM on a substrate. On-substrate growth rates of up to 145 nm per second are reported by Portland State University, for the synthesis of multiple-wall carbon nanotubes, with tube lengths of tens of micrometers, suggesting growth durations of more than one minute. However, these on-substrate growth processes are batch mode processes, and as such, are restricted to relatively low production rates. Substrate preparation is labor intensive and time consuming, as is product collection and refinement. It would be desirable to develop a method and apparatus for producing commercial quantities of such CNMs in a less labor intensive and more efficient manner.
Of the above-described processes, the only continuous production process (the HiPco Process introduced by M. Bronikowski et al.) appears to be limited to a production of 10 g/day (or less than 5 kg/year) of SWCNTs. Such nanotubes are rather short in length compared to other CNMs, which translates to relatively short durations in a temperature-controlled annealing reactor. Continuous-flow methods at production rates of many hundreds of tons per year of product are required to enable large scale introduction of CNMs, and to reduce unit product costs.
It is noted that the purification and separation of mixed CNMs significantly increases the costs of carbon nanotube production. Continuous processing of materials versus batch mode processing (such as the substrate-based CVD process) offers significant cost reduction potential, due to significant increases in production rates, which requires continuous product collection, product removal, separation, and purification (if needed). It would therefore be desirable to develop a method and apparatus for product collection, product removal, and product separation of different CNMs. It would further be desirable to develop a method and apparatus adapted to produce CNMs that do not require a high level of separation and purification.
Inductively coupled plasma (ICP) systems are used in a wide range of applications, including gas spectroscopy, plasma spraying, materials synthesis, waste destruction and waste-to-energy applications (e.g., Vavruska, J. et al., entitled “Induction Steam Plasma Torch For Generating a Steam Plasma For Treating a Feed Slurry” (U.S. Pat. No. 5,611,947), and Blutke, A. et. al., entitled “Use of a Chemically Reactive Plasma For Thermal-Chemical Processes” (U.S. Pat. No. 6,153,852).
Knight, R. et al. have reported isolating carbon nanotubes from residues produced and collected in a reactor energized using an ICP, entitled “Thermal Plasma Process For Recovering Monomers and High Value Carbons From Polymeric Materials” (U.S. Pat. No. 6,444,864). Withers, J. et al., report using a variety of heating devices in the formation of free carbon and fullerene collection in soot particulate in “Methods and Apparati For Producing Fullerenes” (U.S. Pat. No. 5,876,684). This patent emphasizes the use of arc plasma technology, but ICP technology, laser beams, and microwave plasmas are listed as potential heat sources. Neither of these methods discloses in-flight synthesis or continuous product collection and removal. It would be desirable to incorporate such features in an ICP based CNM production process and related apparatus.
A substrate-based method using ICP has been published by NASA Ames Research Center (Delzeit, L. et al., Journal of Appl. Phys., 91:9, (2002)), describing the production of MWCNTs grown on silicon substrates with multilayered Al/Fe catalysts. The authors recognize the benefits of ICP technology for its high ionization efficiency compared to direct current (DC) or radio frequency (RF) capacitive discharges. The process disclosed by NASA operates at very strong vacuum (10−5 Torr) at about 800° C. and at power levels about 500 to 1000 times smaller than is achievable in ICP torches. It would be desirable to develop a process operating at standard atmospheric pressures, which employs a more energetic plasma.
Clearly, new and improved methods that are capable of economically producing large quantities of CNMs are desirable. Such methods should provide consistent product qualities, and be sufficiently flexible so as to be capable of meeting the demands of the market place.